Volcanic Degassing Processes and Atmospheric Impacts

Abigail Metcalfe – Laboratoire Magmas et Volcans

Volcanic eruptions are driven by volatiles in magma, including water (H₂O), carbon dioxide (CO­₂), sulphur (S), chlorine (Cl) and fluorine (F). These volatiles come from different depths within magma plumbing systems, depending on where magma is stored and how it degasses. Magma can hold volatiles in a dissolved form or as gas bubbles, depending on the pressure conditions. When magma cools or undergoes changes, it can become oversaturated with volatiles, leading to the formation of gas bubbles. The build-up of pressure from these bubbles can trigger an eruption. In this way, volatiles can have a significant impact on volcanic behaviour, namely eruptive style, crystal growth and magma ascent.

During an eruption, large quantities of volatiles, in addition to ash and lava, can impact the local environment and even the global climate. Sulphur, in particular, forms sulphate aerosols in the atmosphere, which may result in regional or global cooling. Even during quiet periods between eruptions, volatiles continue to play a role as they passively escape from the magma.

Understanding the processes behind volcanic eruptions and the role of gases in triggering them is crucial for volcano monitoring and crisis management, especially due to the potential for rapid escalation of volcanic activity. This knowledge helps authorities make informed decisions and plan evacuations when necessary.

La Soufrière de Guadeloupe: An Active Volcano in the Caribbean

Guadeloupe is a group of islands located in the Lesser Antilles Arc which formed due to subduction of the American plate under the Caribbean plate (Fig 1). The eastern islands, Grand-Terre and Marie-Galante, are part of the outer arc and are inactive. The western island, Basse-Terre has seven volcanic complexes, with the Grande Découverte Soufrière complex (GDS) hosting the currently active La Soufrière volcano. La Soufrière has been built up through repeated eruptions which produce lava flows, lava domes, and explosive events. Eruptions are triggered by various processes within the magma storage region, such as the injection of new magma from deeper in the system.

The last eruption of La Soufriere was a series of phreatic explosions between July 1976 and March 1977. These explosions caused significant changes in the structure of the volcanic dome (Fig 2). Following this eruption there was a period of quiescence until May 1992 when activity resumed, and a new phase of volcanic unrest began. From this time, seismicity increased, and new fumaroles appeared with an increase in gas emissions. Between April 2017 and December 2018, there was a significant increase in volcanic activity, reaching levels not seen since 1976-1977. Seismicity reached a peak on April 27, 2018, when a magnitude 4.1 earthquake occurred about 3 kilometres below and 2.5 kilometres northwest of La Soufrière. This earthquake was the largest in 42 years. Though the unrest in 2018 did not escalate to an eruption, it is still important to understand what a future eruption could potentially be like. The eruptive history is well documented and detailed study of these eruptions can provide information on the potential of a future eruption.

As part of my PhD work, I studied five past eruptions of La Soufrière covering a range of eruption intensities: [1] 5680 BCE considered one of the largest eruptions in the volcano’s history, [2] 341 CE Eruption produced the Echelle scoria cone, [3] 1010 CE significant explosive phase and dome-building, [4] 1530 CE a partial collapse of the volcano followed by explosive phases and dome formation, [5] 1657 CE most recent magmatic eruption, a smaller but still significant explosive event.

Past Eruptions from Source to Surface

In our recent Frontiers paper, we analyse samples from past eruptions to understand the volatile behaviour at this volcano and provide insight into the processes occurring from source to surface. Through analysis of the volatiles trapped in volcanic glasses (Fig 3) we can understand their behaviour during eruptions and assess the implications for volcanic activity and climate.

Using the volatile contents of the glass, which represents the pre-eruptive volatile content of the magma, we model the volatile behaviour during magma decompression (Fig 4). This allows us to understand the degassing of H₂O, CO₂, and S during magma ascent for eruptions of different intensities.

Our modelling shows that dissolved sulphur is removed from La Soufrière magma through its separation from liquid iron sulphide, prior to degassing. Sulphur’s multiple oxidation states results in variations in the sulphur distribution through the magma allowing sulphur degassing at shallower depths during decompression of different magma batches prior to eruption.

Using melt inclusion and groundmass glass sulphur content and eruption volume, we calculated the volatile emissions for each eruption. Using sulphur dioxide emissions and estimated plume heights for each eruption we were then able to assess the potential climate impact of past eruptions (Fig 5). More intense eruptions (5680 BCE and 1010 CE) released large quantities of sulphur dioxide with eruption columns up to 25 km high reaching into the stratosphere (located 15 km high) and having a more significant impact on climate. These eruptions most likely had local and regional climate effects. The two smaller eruptions studied (1657 CE and 341 CE) where the eruption columns did not reach the stratosphere, did not have a significant effect on global climate. However, all eruptions studied contribute to the background stratospheric aerosol layer variability and global cooling.

Perspectives

To provide current day context for the results of degassing we compare our data to the current degassing behaviour at La Soufrière. The magma system at La Soufrière is open to basaltic replenishment at the base and to gas escape from the top which supplies the hydrothermal emissions observed at the surface. Comparing the present-day measured carbon to sulphur (C/S(total)) ratio to C/S degassing paths calculated in this study shows that the C/S ratio can be well approximated by the input of magmatic C/S ratio from the melt inclusions. This is attained through the continuous flow-through of deep gas from degassing magma at depth. 

Overall, this study contributes to our understanding of volatile processes occurring through the magma system at La Soufrière. We also highlight the potential climate effects of a volcanic eruption at this system. This increases our understanding of volcanism at this system and can be used to improve volcano monitoring.

Abigail is a postdoctoral researcher at Laboratoire Magmas et Volcans (Université Clermont Auvergne) in France. She is currently working on a research project as part of IODP expedition 398 investigating the links and feedbacks between crustal tectonics and volcanic activity using the offshore volcanic record from the Christiana-Santorini-Kolumbo Volcanic Field in Greece.

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